The present invention relates to a method of manufacturing an optical fiber, and more specifically, to a method of accurately and stably measuring tension applied to an optical fiber when it is being drawn.
Generally, when an optical fiber is manufactured, an optical fiber preform is melted by heating from its lower end portion, to pull (draw) the melted and deformed portion thereof into a linear shape. Also, the surface of the optical fiber that is obtained immediately after the drawing by heating, is coated with a thermosetting resin, a UV curing resin or the like. The manufacture of the optical fiber with such heating and pulling means is generally referred to as xe2x80x9cdrawingxe2x80x9d.
In case of obtaining an optical fiber through drawing of an optical fiber preform, if tension applied to the optical fiber during drawing is not controlled, there occurs a change in the transmission characteristics, particularly in transmission loss, of the optical fiber in a longitudinal direction. Thus, it is required that the tension when the optical fiber preform is drawn be controlled so as to be a prescribed value. Specifically, there is a need to measure and control tension applied to the optical fiber being drawn.
In addition, in case of measuring the tension applied to the optical fiber, the optical fiber will be damaged when a contact type measuring device is used. Therefore, a non-contact type measuring device is generally used, to measure the tension applied to the optical fiber.
Here, explanation will be made of a method of measuring tension applied to an optical fiber being drawn. A general method of measuring tension applied to an optical fiber being drawn is a method utilizing oscillation of the optical fiber generated when drawing.
According to this method, target tension T is obtained by acquiring a fundamental oscillation frequency f from a peak of a measured oscillation waveform spectrum, and thereafter substituting the fundamental oscillation frequency f for the following equation (1).
T=(2xc2x7Lxc2x7f)2xc2x7xcfx81xc2x7xcex1xe2x80x83xe2x80x83Equation (1)
In the equation (1) above, L represents a distance between an optical fiber preform and a coating die used for forming a first coating layer on the optical fiber (first coating die). Further, p represents a linear density, and a represents a correction factor.
On the other hand, for enabling measurement of the tension T using the equation (1) above, it is required that the fundamental oscillation frequency f be obtained. As a conventional method, as shown in JP-A-62-137531 (xe2x80x9cJP-Axe2x80x9dmeans unexamined published Japanese patent application), there is known a method in which an oscillation waveform is subjected to the Fourier transform and a fundamental oscillation frequency is determined based on frequency components of the resulting oscillation waveform.
However, in the method disclosed in JP-A-62-137531, in a case where there are spectrum peaks other than that of the fundamental oscillation frequency, it becomes difficult to determine the fundamental oscillation frequency. In an actual case, there arise a peak of oscillation, which is obtained by multiplying the fundamental oscillation frequency by an integer, and a peak which does not correspond to the fundamental oscillation frequency multiplied by an integer. Therefore, it is required that influence caused by those peaks be removed.
Therefore, as shown in U.S. Pat. No. 5,079,433, there is a method of finding a set of spectrum peaks that are close to relation between the fundamental oscillation frequency and a double frequency, from among all of the spectrum peaks.
Further, as shown in JP-A-10-316446, there is a method in which, when performing the first time peak frequency retrieval, the retrieval is conducted within a frequency range where it is expected that a peak frequency is included, for example, in a vicinity of a target value, to thereby perform subsequent peak frequency retrievals in a state where a median value of the peak frequency retrieval is moved to the previously detected peak frequency.
However, in the technique described in U.S. Pat. No. 5,079,433 and JP-A-10-316446, there is fear that error measurement of the fundamental oscillation frequency arises in a case where a noise is a frequency that is close to the fundamental oscillation frequency of the optical fiber or that is close to the frequency obtained by multiplying the fundamental oscillation frequency by an integer, or in a case where there is a noise peak within the peak retrieval range.
Therefore, in the conventional methods, there has been required means for accurately measuring the tension applied to the optical fiber being drawn, by precisely recognizing the fundamental oscillation frequency of the optical fiber regardless of noise generating conditions.